One of the critical parameters involved in the manufacture of paper is its strength; virtually all paper manufactured is sold with a strength specification of some sort, and acceptance of a manufacturer's paper depends on his being able to meet this requirement. Consequently, it would be desirable to measure paper strength "on line" as it is being manufactured, in order to avoid the production of substandard material. Since papermaking is a high speed continuous process, large amounts of paper can easily be produced before strength can be confirmed by a later measurement.
Two problems which make the measurement of strength "on line" difficult arise from the facts that the strength of paper varies across the width of the sheet being produced, and is also different in the machine direction and the cross sheet direction. The system disclosed in my copending application "Sensor and System for Continuous Determination of Paper Strength" (Ser. No. 06/784/213) is intended to allow a continuous determination of strength and its variation across the width of the sheet. However, the sensor used does not have the capability to separate machine direction strength from cross direction strength and therefore does not provide a complete picture of the characteristics of the paper.
Strength specifications for paper are usually given in terms of an empirical destructive test, the more common of these being a standardized tensile test, the so-called "STFI" compression test, and the "burst pressure" or "Mullen" test.
In the standard tensile test, a strip of paper held between two clamps is loaded in tension at a predetermined rate. The loading at failure is taken to be a measure of the tensile strength of the paper. There are a number of standardized procedures which have been adopted to perform this test, e.g., TAPPI Standard T404os-76 and ASTM Standard D828.
The "STFI" compression test for heavy papers is a standardized test whose procedure has been established by the Swedish Technical Forest Institute, as specified by the identifier: Scan P46 Column 83. In this test a strip of the paper to be tested is held between a pair of clamps which are moved together at a fixed rate while the compressive force is monitored. "Rupture" occurs when the compressive force passes the peak and begins to drop. The force at "rupture" is taken as the compressive strength of the paper. Other standard specifications for this test are, e.g., TAPPI 7818os-76 and ASTM D1164.
The strengths of papers as measured by the foregoing tests typically have different values depending on whether the test strip is cut in the machine direction or the cross direction.
A "Mullen" or burst pressure test is conducted by clamping a sample of the paper between two circular clamping rings having a specified standard inside diameter, and building up pressure on one side of the paper until the paper bursts (using a rubber diaphragm and liquid pressure). The pressure required to burst the paper is known as the "burst pressure" and is the figure often used to specify the required strength. Common burst pressure specifications are TAPPI 403os-76 and ASTM D7774.
Needless to say, none of these tests lend themselves to use in connection with the continuous measurement of paper strength. Because of their widespread popularity, however, any method used to measure the strength of paper should provide results which correlate with one of the recognized standard tests.